How Not to Run Hot: Overcoming Thermal Challenges in Wi-Fi Front-End Designs (Part 1)

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November 14, 2017

This
is the first of a two-part blog series that looks at the design challenges for
Wi-Fi front-end designs. Part 2 examines coexistence and
interference.

For wireless access points or customer premises
equipment (CPE), it can be hard to fully account for thermal management
and the parameters affected by it prior to FCC certification. To save yourself
the headaches of last-minute changes due to interference, coexistence or RF
front-end (RFFE) linearity, be sure to design using component thermal
parameters in mind. This blog post explains the biggest thermal challenges
facing Wi-Fi front-end designs.

Increasing capacity for smart
homes

On average, today’s households have 12 clients or Internet of Things
(IoT) products communicating with each other, but these numbers will
increase in coming years.
Intel believes the number of household clients will increase to 50 in
2020, while Gartner
predicts 20.4 billion connected things will be used worldwide by
2020.

In today’s wireless homes, communication operators and retailers have
typically offered one large wireless router, using raw power to achieve
coverage across the entire home. But with the sharp increase of household
devices and the IoT, smart homes are pushing the capabilities of the
single-router model.

As a result, new application models are evolving. Consumers are finding
that placing more routers, or nodes, in the home helps service more clients
and data backhaul to the home router/modem. This mesh (or distributed Wi-Fi) network
model ensures wireless capability across a home using some techniques that
are already present in office headquarters, hospitals and college campuses via
enterprise-level systems.

The IoT challenge

It’s no surprise that the RF complexity within the access point
increases because of this mesh networking model and as devices integrate more
standards and capabilities.

The IoT brings several challenges:

The addition of wireless radio needs. Access points today
incorporate more than just Wi-Fi — they also support Zigbee, Bluetooth, Bluetooth Low Energy
(BLE) and Thread. Operators are also finding ways to reach households who
previously didn’t have access. Operator-supported
LTE-M (the machine-to-machine version of LTE) is one example finding its
way into some Wi-Fi gateways.

More users within each home. Homes no longer have only
one or two PCs and a few phones. Today, several computers, TVs, smartphones,
wearables, security networks, wireless appliances and more all connect to
Wi-Fi and the internet.

Additional Wi-Fi bands. Units no longer have one 2.4 GHz
band and one 5 GHz band. Now there are up to eight individual 2.4 GHz
and eight 5 GHz paths. This change gives us the MIMO (multiple input / multiple
output) and multiuser MIMO (MU-MIMO)
paths within the Wi-Fi access point or node.

Shrinking size and expanded functionality. Wi-Fi
manufacturers are making Wi-Fi units smaller, sleeker, more decorative and
less obtrusive. They’re also making some units all-weather or adding
multifunctionality such as night-light capability.

Block diagrams of older vs. new access points highlight just how complex
the RFFE design is now.

Running hot

All these changes in the Wi-Fi front-end design increase the number of RF
chains, contributing to the overall heat within the access point. This
increase in unit temperature also increases the RF tuning challenges,
especially when the size of the box is the same or even smaller.

In the Wi-Fi world, one of the most critical design challenges engineers
need to address is product temperature. In today’s products, components
are subject to average temperatures of 60°C or greater, while sitting in a
room temperature environment of 25°C. It’s important to consider
this fact early on in a design, to help minimize redesign issues or additional
costs.

How heat challenges RF front ends to
deliver capacity and range

Temperature affects three RFFE components:

Power amplifiers

RF switches and low noise amplifiers (LNAs)

Filters

Let’s examine the heat challenges and Wi-Fi design considerations for
each category.

In the Wi-Fi world, one of the most critical design challenges engineers
need to address is product temperature.

#1: How does the power amplifier
factor in?

Engineers often balance between linearity, power output and efficiency in
each of the RF chains. Using optimized, highly linear power amplifiers or
front-end modules (FEMs) optimizes system efficiencies, creating less overall
heat. It also reduces processing inefficiencies.

RF engineers should also consider several Wi-Fi design trends that affect
power amplifiers:

Use of time division duplexing (TDD). Wi-Fi networks
use TDD, which means the PAs are pulsed on and off during operation —
alternating transmit and receive functions. This increases PA transients,
which attribute to high temperatures.

More difficult error vector magnitude (EVM)
specifications. EVM is a measure of modulation quality and error
performance. In 802.11ac, the EVM specification was -35 dB, but in
the
next standard of Wi-Fi, 802.11ax, this specification increases to
-47 dB — which is more difficult for Wi-Fi component designers
to meet. Design engineers must design highly linear FEMs to optimize for
EVM, which ultimately helps reduce overall product temperature.

Higher modulation schemes. To help achieve higher
capacity and data rates, Wi-Fi designs are moving from 256 QAM to
1024 QAM modulation schemes. With 1024 QAM modulation, there are
more bits per symbol — 10 bits per symbol versus 8 bits in
256 QAM. However, as the data rate increases, EVM on the RFFE becomes
a principal concern. The constellation is so dense in 1024 QAM that
the processor must use sophisticated system decoding to distinguish each
point. When the processor works harder, the unit device heat
increases.

How the RFFE performance affects the overall current draw on
the system processor. Poor RF front-end performance means the
processor will have to work harder to meet overall system requirements.
Working the processor more increases system hardware heat.

#2: What about the RF switch and low
noise amplifier (LNA)?

In the switch, insertion loss can also generate excess heat. When insertion
loss increases and signal strength is lowered, the PA works harder to
compensate and push higher outputs, which degrades efficiency. And less
efficiency means more heat from the device. Using high-linearity, low-loss
switches keeps the insertion loss within specifications across the entire
band.

Receive throughput is highly dependent on the LNA gain and noise figure.
Although the LNA doesn’t contribute significantly to heat generation,
the effect of heat on the LNA can drastically affect throughput. Heat degrades
the noise figure, and depending on the circuit design and choice of wafer
technology, the compensation for this can lead a designer to a specific
solution.

#3: Finally, the filters

RF filters drift to the left or the right due to changes in temperature,
as shown in the following SAW vs. BAW figure. These shifts can cause high
insertion loss on the band edges, which could cause a low gain or
POUT response from the RFFE. If the filter drifts too much (as
shown in the SAW figure), then the PA pushes more power output to compensate
for the insertion loss. This increases current and decreases system
efficiency.

Using filters with high insertion loss can decrease linearity and
increase the RF chain POUT. One big advantage of Qorvo’s
LowDrift™ bulk acoustic wave (BAW) filters is their stability over
temperature shifts. Diplexers, bandpass filters and coexistence filters that
use BAW technology with lower temperature drift help mitigate insertion
loss, and lead to good product thermals.

Design considerations for power
consumption: Qorvo’s approach

Heat can degrade overall system performance (such as throughput, range
and interference resolution). As a result, it’s important to design
systems by choosing RFFE components that mitigate the heat. In the transmit
chain, the focus should be on balancing link budget needs such as system
linear power.

As devices move from 802.11ac to 802.11ax capability, product
manufacturers must focus on using more efficient components. Qorvo
challenges its design teams to increase linear power without increasing
power dissipation — designing higher throughput devices with the same
power consumption as previous generations. One example is the QPF4528, an 802.11ax 5 GHz FEM that transmits linear powers achieving
-47 dB EVM — above the current QPF4538 FEM, an 802.11ac 5 GHz FEM that
achieves ‑43 dB EVM with the lower maximum power dissipation.

Another product that integrates all the aspects of heat mitigation is
Qorvo’s QPF7200, a fully integrated
front-end module (iFEM) that reduces weight and design complexity while also
decreasing system heat. The QPF7200 module:

Contains a highly efficient 2 GHz power amplifier to reduce
heat

Integrates an FCC bandedge LowDrift BAW filter, which is resistant to
temperature shifts and provides the option to remove the number of required
RF chains

Includes an LTE Wi-Fi coexistence filter that negates the interference
effects of LTE devices such as phones or modems, which could degrade
throughput

Think about operating temperatures
before FCC certification

With so many radios and RF chains squeezed together, it’s important
to partner with an RF supplier that helps you achieve low power dissipation
and linear power budgets simultaneously.

Although many systems are designed and modeled at room temperature, ask
yourself how it will operate at 60-70°C (140-158°F) when the device is
operating. Don’t wait until FCC certification time to find out.

Check out Part 2 in our series, where we address Wi-Fi
design challenges around wireless interference/coexistence, and smart tips to
overcome them.

Have another topic that you would like Qorvo experts to cover? Email your
suggestions to the Qorvo Blog team and it
could be featured in an upcoming post.

About the Author

Wayne PolonioSenior Product Marketing Manager

With 18+ years in the RF industry, Wayne helps customers around the world overcome the design challenges associated with the growing complexity of Wi-Fi applications.